Not Applicable.
Not Applicable.
1. Field of the Invention
The present invention relates generally to a silencer and method for use with an ultrasonic meter that reduces noise in the ultrasonic range of frequencies generated by other equipment in the flow stream. More particularly, the invention relates to a silencer and method for use with an ultrasonic meter that is capable of reducing ultrasonic noise under high-pressure operating conditions. Still more particularly, the invention relates to a silencer and method for use with an ultrasonic meter that also acts as a reasonable flow conditioner.
2. Background of the Invention
In pipeline operations and other industrial applications, meters must be capable of accurately measuring the flow rate of gases or liquids moving through piping or tubing systems. In natural gas pipelines, for example, these flow rate measurements may be relied upon for custody transfer, leak detection, control, or for other indications.
For custody transfer operations, the meter is the point where custody transfer occurs, such as when gas is delivered into or out of a pipeline system through the meter as it measures the passing flow rate. By accurately measuring the flow rate for a given time period, the volume of gas that passes through the meter can be determined, and a custody transfer volume ticket can then be prepared. The pipeline transportation fee is based on the volume of product moved through the system, i.e. the custody transfer volume. Thus, a custody transfer metering system is commonly referred to in the pipeline industry as the xe2x80x9ccash register,xe2x80x9d and pipeline operators take great care to maintain its measurement accuracy.
Measurement systems comprising two or more meters may perform a pipeline leak detection function. A pipeline typically operates in a xe2x80x9cpackedxe2x80x9d or full-line condition. Therefore, as gas is pumped into the system through the inlet meter, gas is simultaneously delivered out of the system through the outlet meter, and the measurements taken at each meter are compared. This xe2x80x9cmeter-in, meter-outxe2x80x9d approach provides two modes of leak detection. First, the flow rate measured by the inlet meter should match the flow rate measured by the outlet meter within a certain accuracy tolerance, taking into account characteristics that may cause flow rate deviations, such as elevation differences or product temperature variations. Second, by measuring flow rate over a given time period, the volume moved through each meter can be determined, and the inlet and outlet meter volume measurements should correlate over that time period. A measurement discrepancy could indicate a pipeline leak, although the storage of gas in the pipeline (line packing) makes short-term leak measurements difficult. Nonetheless, early leak detection enables a pipeline operator to locate and repair the problem more quickly, thereby minimizing the environmental and public safety impacts of a leak. Thus, accurate metering systems are necessary for profitable, safe, and reliable pipeline system operations and other industrial applications.
Flow meters are available in many different forms. Most conventional meters, such as turbine meters, are inserted directly into the flow stream where the gas drives a rotor mounted within a meter housing. The meter measures the number of rotations per unit time, which is proportional to the gas flow rate. These meters are fairly expensive and require regular calibrations to maintain accuracy over a long time period. They are also intrusive to the flow stream and include moving parts with close internal tolerances that are susceptible to damage from gas flow stream contaminants.
The ultrasonic meter is often a preferable metering device in gas flow streams because it overcomes the problems of conventional in-line meters by measuring flow rate in a non-invasive fashion, with considerable accuracy, and with no moving parts. An ultrasonic meter includes two or more transducers that emit ultrasonic waves into the flow stream and measure the propagation time of each wave to determine the flow rate of the passing gas stream. An ultrasonic wave is a sound wave having a frequency above the audible sound range, and more particularly, having a frequency  greater than 20 kHz. A typical ultrasonic meter emits ultrasonic waves at frequencies between 50 kHz and 300 kHz, and preferably between 80 kHz and 180 kHz. U.S. Pat. No. 4,646,575 (hereby incorporated herein by reference for all purposes) discloses an ultrasonic meter and many of its features.
An ultrasonic measurement system may include a silencer placed between the meter and other equipment in the measurement flow stream. The silencer reduces stray ultrasonic noise that interferes with the accuracy of the ultrasonic meter. Such stray ultrasonic noise is commonly produced during gas distribution where the gas pressure is dropped precipitously and generates substantial noise (i.e. enough to interfere with measurements). A pressure-regulating valve that reduces the pressure of multiple incoming flow streams as the gas is combined into a common supply pipeline, or reduces the pressure from a main supply grid to local distribution, is another source of ultrasonic noise. Environmental regulations set upper limits on the acoustic noise level that industrial equipment can emit. To avoid excess acoustic noise, a pressure-regulating valve may be designed, for example, to reduce gas pressure by variably restricting small holes drilled into a rigid steel plate to reduce, as far as possible, the emission of sound waves in the acoustic range of frequencies. However, because the gas flow approaches supersonic velocity as it moves through these drilled holes, the pressure regulating valve instead generates high levels of broad band ultrasonic noise. This ultrasonic noise propagates through the gas to interfere with the ultrasonic flow meter signals, resulting in a poor signal to noise ratio and a loss of measurement accuracy.
Silencers are designed to attenuate the wave energy of stray ultrasonic noise by reflection, absorption or both. PCT Application WO 97/31365 (the contents of which are hereby incorporated herein by reference for all purposes) discloses one type of ultrasonic silencer that uses a diffuser arrangement, such as a perforated tubular body, with a multiplicity of small-area surfaces that frequently reflect the ultrasonic waves. These reflections result in destructive interference between the acoustic paths, thereby effectively damping the ultrasonic noise. The noise is attenuated by scattering the ultrasonic energy from the wave and reflecting it in many different directions. The diffuser surfaces of the silencer are preferably at least partially curved, leading to the formation of vortices inside the gas flow that likewise cause acoustic path interference to reduce the ultrasonic noise. These gas vortices can introduce undesirable flow disturbances into the measurement path, thus requiring the silencer to be located a minimum distance away from the meter. This distance requirement may be undesirable when space is limited.
A second type of silencer relies on absorption to attenuate stray ultrasonic noise. This silencer is a foam plug, formed of an open-cell material that is inserted into the flow stream for the gas to pass through before entering the measurement flow path. The foam plug attenuates noise by converting the ultrasonic energy into thermal energy through friction loss in the interstices of the material. Although this is an effective ultrasonic silencer, high-pressure loss is observed as the gas flows through the foam plug. Furthermore, the foam plug acts to filter dirt and absorb liquids in the flow stream. Thus, the open-cell foam plug silencer is only suitable for use in clean gas service and in systems that can accommodate high pressure loss through the silencer.
French Publication No. 2,737,564 (the contents of which are hereby incorporated herein by reference for all purposes) discloses another type of silencer that relies on both absorption and reflection to attenuate stray ultrasonic noise. This type of silencer includes a chamber with walls formed of a closed-cell, visco-elastic, absorbing material that is porous, such as, for example, a polyurethane foam, with a pore size chosen to absorb the unwanted ultrasonic waves at a particular frequency to achieve the desired attenuating effect. The absorbing material may be flexible or rigid. When ultrasonic waves enter the chamber, they are partially reflected off the absorbent material of the walls to attenuate the amplitude of these waves. To increase the attenuating effect of reflection, the silencer may include projecting walls that form passages to trap the ultrasonic waves, forcing multiple reflections and energy loss (by absorption) upon each reflection. Another way to attenuate the ultrasonic waves by reflection is to place an obstacle, formed of either absorbing or reflecting material, internally of the chamber between the inlet and outlet of the meter. The obstacle splits the flow stream and forces the waves to be reflected many times as the waves move between the chamber inlet and outlet. Silencers of this type are effective for use with ultrasonic flow meters operating at essentially atmospheric pressures of approximately 1 bar. However, a closed cell, visco-elastic foam exhibits an acoustic performance that decreases with increasing pressure. Namely, these closed-cell foam materials do not perform well at high pressures (up to 400 bar) because they tend to compress, thereby reducing the thickness and void fraction of the material to significantly reduce the sound absorbing quality of the foam.
Thus, to overcome deficiencies associated with prior silencers, it would be desirable for an ultrasonic silencer to be comprised of an absorbing material capable of maintaining its absorbing characteristics under high pressure operating conditions up to 400 bar. Further, it would be advantageous to have an ultrasonic silencer configured to introduce only a low pressure drop to the system. Additionally, it would be desirable to have an ultrasonic silencer that is suitable for use in either clean or contaminated gas service. It would also be desirable to have an ultrasonic silencer that introduces no flow disturbances, such as vortices, into the measurement path, but rather acts as a reasonable flow conditioner, thereby allowing the silencer to be bolted directly to the meter to minimize equipment space requirements.
The present invention features a silencer for use with an ultrasonic meter to reduce ultrasonic noise that would otherwise interfere with the meter and cause measurement inaccuracies. For effective use, the silencer should be mounted between the noise source and the ultrasonic meter. Thus, depending upon the location of the noise source, it may be mounted either upstream or downstream of the meter. When located upstream of the meter, it also acts as a reasonable flow conditioner. It therefore can be mounted directly to the meter in either an upstream or downstream position without introducing flow disturbances into the measurement flow path.
The silencer comprises a tubular body having at least two partitioning members or baffles internally disposed therein, with the width of each baffle disposed perpendicular to the flow and the length of each baffle disposed parallel to the flow. The baffles are formed of an open-cell material designed to absorb noise in the ultrasonic range of frequencies under high-pressure operating conditions, and even more preferably the baffles are formed of a reticulated metal foam. The baffles are flat plate members, or in another embodiment, concentric cylindrical members, or in yet another embodiment, corrugated plate members, spaced apart one from another to partition the flow area into discrete passageways. As ultrasonic noise waves enter the silencer, the waves propagate through the flow passageways and reflect between the baffles. With each reflection, the a small quantity of ultrasonic wave energy is absorbed by the baffle material, thereby attenuating the ultrasonic noise level.
Thus, embodiments of the present invention comprise a combination of features and advantages that enable it to overcome various problems of prior silencers. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.